Why does a capacitor blocks DC but passes AC?

Picture this: a capacitor is like a bouncer at an exclusive club. Direct current (DC) is like a steady stream of partygoers trying to get in. The capacitor says, "No way, Jose! I'm not letting you in unless you're on the VIP list." And since DC is a constant voltage, it's not on the VIP list, so it gets blocked.

Now, alternating current (AC) is a whole different story. It's like a group of partygoers who are doing the wave, jumping up and down in a pattern. The capacitor says, "Hey, you're doing the wave! You're on the VIP list! Come on in!" And since AC is a changing voltage, it's on the VIP list, so it gets to pass through the capacitor.

In technical terms, a capacitor blocks DC because it has a high reactance to DC voltage, which means it resists the flow of direct current. On the other hand, it passes AC because the changing voltage of AC causes the capacitor to charge and discharge, allowing the current to flow through.

So, to sum it up, a capacitor is like a picky eater who only lets the cool kids (AC) in, while keeping the boring ones (DC) out.

Now let's discuss it more technically:

At the heart of this phenomenon lies the concept of impedance. Impedance is the total opposition that a circuit presents to an alternating current (AC) or voltage. It is composed of two parts: resistance (R) and reactance (X). Resistance is the opposition to direct current (DC), while reactance is the opposition to AC.

In a capacitor, the reactance is known as capacitive reactance (XC). The formula for capacitive reactance is:

• XC = 1 / (2πfC)

Where:

• XC is the capacitive reactance in ohms

• f is the frequency of the AC signal in hertz (Hz)

• C is the capacitance in farads (F)

• π is approximately 3.14

When a DC voltage is applied to a capacitor, the frequency (f) is 0 Hz. Plugging this into the formula, we get:

• XC = 1 / (2π * 0 * C) = 1 / 0 = infinity

This means that the capacitor presents an infinite impedance to DC, effectively blocking it.

On the other hand, when an AC voltage is applied to a capacitor, the frequency is non-zero. As the frequency increases, the capacitive reactance decreases, allowing the AC to pass through the capacitor. This is because the capacitor charges and discharges with the changes in the AC voltage, allowing the current to flow.

In summary, a capacitor blocks DC because it presents an infinite impedance to it, while it passes AC because the capacitive reactance decreases with increasing frequency, allowing the current to flow.